Dental resin composite restorations break down due to recurrent caries and restoration defects, such that approximately half or more fail within about 10 years. In the case of bisphenol-A glycidyl dimethacrylate (Bis-GMA)-based resins, their breakdown product can be bisphenol-A, a known health hazard. Furthermore, in Bis-GMA and urethane dimethacrylate (UDMA) based resins, leaching of both unreacted monomers and degradation products stimulate oral bacteria to produce even more acid byproducts and esterases, all of which could lead to further ongoing degradation and possibly secondary caries in the nearby tooth structure. In order to extend the clinical lifetime of dental resin based composites, their buildup of shrinkage stress, the susceptibility of their ester groups to hydrolysis and esterase degradation, and the formation of marginal gaps between the resin and tooth structure must be overcome.
Thus, the inventors have developed a hydrophobic, hydrolysis and esterase-resistant, dental restorative system based on Oxirane/Acrylate Systems (OASys, pronounced “Oasis”). The OASys contains the following: monomers with methacrylate endgroups that are replaced with a combination of monomers containing oxirane (a.k.a. epoxy) or acrylate endgroups that form an interpenetrating polymer network (IPN), and a tri-functional bonding agent with oxirane, acrylate and phosphate functionality that is capable of bonding to both networks in the IPN as well as the tooth structure. Alternatively, a bonding agent with diepoxide or diacrylate and phosphate functional endgroups can also be used.
Preferably, both types of monomers in the IPN are multifunctional, e.g., dioxirane and diacrylates, based on urethane monomers because urethane linkages are more resistant to hydrolysis and esterase degradation than ester linkages, and may also be fluorinated to further increase hydrophobicity and decrease hydrolysis and esterase degradation. Such components polymerize separately, but simultaneously, to form independent networks that are each highly converted and crosslinked, but physically intertwined. Such IPNs offer synergistic advantages that provide superior performance as restorative dental composites via the following mechanisms.
These IPN resins can provide higher-levels of mechanical and physical properties as compared to methacrylate-based resins, such as increased toughness. They also have lower residual cure-shrinkage stresses since epoxy monomers polymerize via a ring opening mechanism reducing cure shrinkage, and their polymerization is substantially slower allowing more time for stress relaxation. The acrylate network cures quickly and allows the dentist to work with a hardened structure while the oxirane cures over a longer period of time. Acrylic and oxirane functionality are also significantly more resistant to hydrolytic and enzymatic degradation than methacrylic functionality, which increases longevity. Furthermore, methacrylate polymerization is inhibited by oxygen, while acrylate polymerization is faster and less susceptible to oxygen inhibition and oxiranes are not susceptible to oxygen inhibition. Therefore, the surface layer of an OASys resin will suffer little from oxygen inhibition and thus have a higher degree of conversion and crosslinking, making it more resistant to water imbibition and subsequent hydrolysis.
Such oxirane/acrylate hybrid resin systems and the properties of IPNs have been described in the literature in different forms. However, these systems are not currently used as dental restoratives. Reasons include the toxicity of the amines typically used with epoxy resins, the toxicity of cationic byproducts of epoxy polymerization, and the lack of an adequate dental bonding system for such a hybrid resin system. The issue of cationic byproduct toxicity has largely been addressed with the use of dual-mode light-cure initiator systems that use polyols to quench the cationic byproducts. Another reason is that there does not exist an adequate dental bonding system that is capable of attaching to both parts of the IPN as well as the tooth structure. The use of a conventional bonding agent to attach the tooth structure to only the acrylate or methacrylate network may actually weaken the restorative since the other network is not attached at all.
The invention includes a series of bonding agents. One bonding agent contains a phosphate group plus both oxirane and acrylate functionalities. FIG. 16 shows an example of 4-Phospho-NPG GA oxirane (4POA). The phosphate group forms a bond to the hard tooth tissue (enamel and dentin), and both oxirane and acrylate functional groups bind, respectively, to the epoxy and acrylate polymer networks, and thereby attach the OASys restorative composite at the composite/hard tissue interface. Alternatively, a mixture of a diepoxide bonding agent with a phosphate endgroup (FIG. 18) or a diacrylate bonding agent with a phosphate endgroup (FIG. 19) can be used. Additionally, these bonding systems are one-step (primer-less), “smart,” antimicrobial bonding resins with in situ-generated silver nanoparticles (AgNPs) that is capable of releasing antimicrobial Ag+ ions in the event of marginal gap formation to prevent secondary caries. These bonding agents can further increase bond strength and prevent marginal gap formation.